docs: def label(self) -> str:
          """A latex formatted label representing constant expression and united value."""
          label = self.expression.replace("_", "\\;")
          if self.units_kind:
              symbol = wt_units.get_symbol(self.units)
              for v in self.variables:
                  vl = "%s_{%s}" % (symbol, v.label)
                  vl = vl.replace("_{}", "")  # label can be empty, no empty subscripts
                  label = label.replace(v.natural_name, vl)
                  val = (
                      round(self.value, self.round_spec)
                      if self.round_spec is not None
                      else self.value
                  )
          label += r"\,=\,{}".format(format(val, self.format_spec))
          if self.units_kind:
              units_dictionary = getattr(wt_units, self.units_kind)
              label += r"\,"
              label += units_dictionary[self.units][2]
          label = r"$\mathsf{%s}$" % label
          return label